This invention relates to a semiconductor memory device having a protecting function against illegal copying, and more particularly to an electrically programmable mask ROM having redundancy cells and a method for protecting the same from illegal copying.
The memory cell array of a semiconductor memory device, for example, a mask ROM (Read Only Memory) is constructed by arranging memory cells formed of MOS transistors in a matrix form, connecting the gates of the memory cells on each row to a corresponding one of a plurality of word lines extending in the row direction, connecting the drains of the MOS transistors on each column to a corresponding one of a plurality of bit lines extending in the column direction, and grounding the sources of the MOS transistors. As a method for writing data into the memory cell, (a) a diffusion layer programming system, (b) ion-implantation programming system, (c) contact programming system and the like are known, information of "0", "1" is set to correspond to the presence or absence of a MOS transistor, the depletion type or enhancement type of a MOS transistor, or the presence or absence of a contact hole, and data is written by using a photomask in the course of the manufacturing process. The operation of reading out stored data is effected by selecting and charging one of the bit lines, selecting one of the word lines and setting the same to a high potential level, and determining "0" or "1" of stored information according to whether or not the bit line is discharged via a MOS transistor (memory cell) which is connected to both of the selected bit line and selected word line.
Generally, in the above mask ROM, redundancy cells which are used instead of defective memory cells when the defective memory cells are present are provided to relieve the memory. The redundancy cell is constructed by an electrically programmable memory element and data for the defective memory cell is stored therein. When the defective cell is accessed, data is read out from the redundancy cell instead of the defective cell, thus compensating for the defect of the cell.
However, with the above construction, data stored in the memory cell array can be easily read out by sequentially incrementing the ROM address by use of a ROM writer or personal computer. Therefore, there occurs a problem that the stored data can be easily copied by writing readout data into a storage medium such as a hard disk or floppy disk.
Therefore, various types of circuits and copy protect methods for providing a protect function against the illegal copying are proposed. For example, the techniques disclosed in Jap. Pat. Appln. KOKAI Publications Nos. 1-173244, 2-42551, 4-74240 and 7-262000 are known. However, if an attempt is made to attain a sufficient protection effect, a problem that the circuit scale is increased occurs.